Lenses produce non-uniform exposure at their focal plane when imaging a uniformly lit surface. For instance, the light from a uniformly lit gray wall perpendicular to the optical axis will pass through a lens and form an image that is brightest at the center and dims radially. When the lens is an ideal thin lens, the intensity of light in the image will form an intensity pattern described by cos4 of the angle between the optical axis of the lens and the point in the image plane. The visible effect of this phenomena is referred to as falloff.
In addition, other factors such as flash falloff and vignetting contribute to the falloff phenomena, which is often visible in an image. Vignetting is a property that describes the loss of light rays passing through an optical system. Flash falloff is a phenomenon inherent in the directional illumination of a nearby, artificial illuminant.
Several examples exist in the prior art which teach methods of compensating an image for the falloff that occurred at the time of capture. In U.S. Pat. No. 5,461,440, Toyoda et al describe a method of recording a camera identification code onto the film upon which the image is also captured. This identification code specifies the lens information (focal length, focus position, and aperture value). During digital processing, the identification code is translated by a look-up-table to a required level of correction which is applied to the image.
However, it is not always practical or possible to record such information onto photographic film. Consequently, the need exists to compensate for levels of lens falloff in a captured scene with less knowledge about the camera's optical system at the time of image capture.
Additionally, in commonly assigned, copending U.S. Ser. No. 09/293,197 (“A Method for Compensating Digital Images for Light Falloff and an Apparatus Therefor”), which is incorporated herein by reference, Gallagher and Gindele describe a method for applying a falloff compensation to a digital image. The compensation is performed by determining an individual compensation value for each pixel of the digital image, based upon a falloff compensation function and a falloff correction parameter. However, while the falloff compensation taught by Gallagher and Gindele successfully corrects for the falloff in the digital image, the corrected digital image consistently appears lighter then the original digital image. This is an undesirable side effect of the compensation. For example, if a digital image which appeared to have the proper balance were compensated for falloff, the compensated digital image would appear too light. Furthermore, the degree of the lightness of the corrected digital image is dependent upon the falloff compensation function and the falloff correction parameter.
Consequently, a need exists for overcoming the above-described drawbacks. More specifically, a need exists for applying a falloff compensation to a digital image in such a manner that the balance of the compensated digital image is similar to that of the original digital image.